This invention relates to a hydraulic braking apparatus of the type comprising, for the actuation of the wheel brakes:                a service braking system, supplied with a pressure brake fluid by a central hydraulic unit, using an external energy source;        an emergency braking system, controlled by muscular energy;        a hand-control member, the forward travel of which actuates the service braking system or, in the case of a failure of the latter, the emergency braking system;        A master cylinder having at least one primary piston, the stroke of which is controlled by the hand-control member;        at least one safety valve, capable either of separating the master cylinder from the wheel brakes when the service braking system operates properly or, should the service braking system fail to operate correctly, of connecting the master cylinder with at least one wheel brake;        a feeling simulator, intended to resist the forward motion of the hand-control member with a reaction corresponding to the progress of a braking operation, such simulator comprising a cylinder in which a simulator piston may slide while being subjected, in one direction, to a fluid pressure from the master cylinder and, in the opposite direction, to a counterforce dependent on the travel of the hand-control member;        pressure-fluid admission solenoid valves and exhaust solenoid valves, connected to the wheel brakes;        sensors for the detection of various braking parameters, in particular the travel of the hand-control member, and the pressures at various spots of the apparatus;        and a computer, connected to the various sensors and capable of controlling the solenoid valves so as to obtain the desired pressures in the wheel brakes.        
A braking apparatus of said type is known, for instance, from FR 2 772 706 or from U.S. Pat. No. 5,544,948.
In such an apparatus, in the course of a trouble-free operation in the service braking mode, the master cylinder is isolated and the fluid, contained in the master cylinder, cannot flow back to the wheel brakes. The hand-control member, e.g. a brake pedal or a hand-brake lever, retains a normal actuating travel, dependent on the exerted force, thanks to the feeling simulator, which comprises a cylinder connected to the master cylinder for the fluid transfers.
The well-known apparatuses operate satisfactorily and, besides, they make it possible to lay down a law of variation for the force to be applied to the hand-control member as a function of the travel, which may give the driver a feeling like that he would get if the pressure inside the wheel brakes resulted directly from the pressure supplied by the master cylinder, and from the muscular force exerted on the brake pedal.
Yet, in these apparatuses known per se, the law of variation concerning the force to be applied to the hand-control member is somewhat fixed, and it cannot be altered in a simple and rapid manner.
Now then, on various grounds, more especially depending on the type of the motor vehicle concerned, it is most desirable that said law of variation should be alterable as simply and as rapidly as possible.
Besides, it is most desirable that the simulator should consume as little fluid as possible, so that the emergency braking, achieved with the help of the master cylinder, may remain as efficient as possible.